β-type titanium alloys are titanium alloys to which V, Mo, or other β-type stabilizing elements are added to retain a stable β-phase at room temperature. β-type titanium alloys are superior in cold workability. Due to precipitation hardening of a fine α phase during aging heat treatment, a tensile strength of a high strength of approximately 1400 MPa is obtained and the Young's modulus is relatively low, so the alloys are used for springs, golf club heads, fasteners, and various other applications.
Conventional β-type titanium alloys contain large amounts of V or Mo such as a Ti-15 mass % V-3 mass % Cr-3 mass % Sn-3 mass % Al (hereinafter, “mass %” omitted), Ti-13V-11Cr-3Al, and Ti-3Al-8V-6Cr-4Mo-4Zr. The total amount of V and Mo is 12 mass % or more.
As opposed to this, β-type titanium alloys in which the amounts of addition of V and Mo are suppressed and the relatively inexpensive β-type stabilizing elements of Fe and Cr are added have been proposed.
The invention described in Japanese Patent No. 2859102 is a Ti—Al—Fe—Mo-based β-type titanium alloy which has an Mo eq (Mo equivalent) larger than 16. A typical composition is Al: 1 to 2 mass %, Fe: 4 to 5 mass %, Mo: 4 to 7 mass %, and O (oxygen): 0.25 mass % or less.
The inventions described in Japanese Patent Publication (A) No. 03-61341, Japanese Patent Publication (A) No. 2002-235133, and Japanese Patent Publication (A) No. 2005-60821 are Ti—Al—Fe—Cr-based β-type titanium alloys in which V and Mo are not added and in which, by mass %, Fe is in a range of 1 to 4%, 8.8% or less (however, Fe+0.6Cr is 6 to 10%), and 5% or less, respectively and Cr is in a range of 6 to 13%, 2 to 12% (however, Fe+0.6Cr is 6 to 10%), and 10 to 20%, respectively.
The inventions described in Japanese Patent Publication (A) No. 2005-154850, Japanese Patent Publication (A) No. 2004-270009, and Japanese Patent Publication (A) No. 2006-111934 are respectively Ti—Al—Fe—Cr—V—Mo—Zr-based, Ti—Al—Fe—Cr—V—Sn-based, and Ti—Al—Fe—Cr—V—Mo-based β-type titanium alloys. In each, Fe and Cr are both added and both or either of V and Mo are included. Furthermore, in Japanese Patent Publication (A) No. 2005-154850 and Japanese Patent Publication (A) No. 2004-270009, respectively, 2 to 6 mass % of Zr and 2 to 5 mass % of Sn are added.